Astatine

Astatine is the heaviest halogen, sitting below iodine in the periodic table. It is so radioactive that a visible lump would vaporize itself with its own heat, so no one has ever seen it. Traces form naturally as uranium and thorium decay, but less than a gram is thought to exist in Earth's crust at any moment, making it the rarest naturally occurring element.

Dale Corson, Kenneth MacKenzie and Emilio Segrè first made it in 1940 at Berkeley by firing alpha particles at bismuth. The name comes from the Greek astatos, unstable. Its longest-lived form lasts only about eight hours. Astatine-211 is being tested in cancer treatments, carried to tumors on molecules that seek them out.